Sarcolemma and myofibrils have been isolated from cardiac muscle or normal (RB) and cardiomyopathic (BIO 53.58) hamsters. (Na ion plus K ion) ATPase and adenyl cyclase were measured in the sarcolemma and calcium-sensitive magnesium ATPase was assayed in the myofibrils. The enzymes did not change to a great extent in the sarcolemma and myofibrils from the normal hamster. However, it was found that the (Na ion plus K ion) ATPase, adenyl cyclase and the magnesium ATPase were decreased at 1 month, increased at 2 months and decreased at three months of age as compared to the normal hamsters in the female cardiomuopathic hamsters. There was no change in the male cardiomyopathic hamsters at one month, slight increase at 2 months and a definite decline at 3 months of age in the enzyme activities. In vitro experiments with normal sarcolemma and myofibrils indicated that the variations in the enzyme activities may be due to alterations in the norepinephrine and calcium content in the cardiac muscle of the cardiomyopathic hamsters.